1. Field of the Invention
The invention relates to a double-pipe-structure hollow member, a method of manufacturing a double-pipe-structure hollow member, and a fluid treating system employing a double-pipe-structure hollow member. More specifically, the invention relates to a double-pipe-structure hollow member composed of an inner pipe and an outer pipe, a method of manufacturing such a double-pipe-structure hollow member, and a fluid treating system that employs a double-pipe-structure hollow member composed of an inner pipe and an outer pipe as a fluid-treating-member container and that holds a fluid treating member inside the fluid-treating-member container.
2. Description of Related Art
A catalytic converter is provided in an exhaust system of a combustion system such as an internal combustion engine to remove noxious substances from exhaust gas flowing therethrough as fluid. In general, as shown in FIG. 9, a catalytic converter has a catalyst 102 as a fluid treating member provided via a mat 103 inside a catalyst container 101, which is a hollow member through which exhaust gas flows. The catalyst container 101 has a catalyst installing portion 101a formed at a central portion thereof and cylindrical connecting portions 101c formed at both ends thereof. The connecting portions 101c are connected with an inflow pipe extending from an internal combustion engine and an outflow pipe extending to a post-treatment portion such as a muffler. While the catalyst installing portion 101a has a relatively large diameter, the connecting portions 101c have a relatively small diameter. Tapered cone portions 101b with a gradually decreasing diameter are formed between the catalyst installing portion 101a having the relatively large diameter and the connecting portions 101c having the relatively small diameter.
As is disclosed e.g. in Japanese Patent Application Laid-Open No. 11-336537, Japanese Patent Application Laid-Open No. 2000-94050, or Japanese Patent Application Laid-Open No. 6-101465, it has been known to provide a catalyst container of such a catalytic converter with a double-pipe structure for the purpose of maintaining the catalyst at a certain temperature and improving exhaust-gas purification performance. As is also disclosed in Japanese Patent Application Laid-Open No. 11-336537, a double-pipe-structure catalyst container generally has a construction wherein an inner pipe and an outer pipe are disposed concentrically with a radial air gap left therebetween.
Japanese Patent Application Laid-Open No. 11-336537 discloses an exhaust-system component composed of an inner pipe having substantially tapered reduced-diameter portions at both ends thereof and an outer pipe having substantially tapered reduced diameter portions at both ends thereof and covering the inner pipe along its entire length including the reduced-diameter portions at both the ends with an air gap left therebetween, characterized in that at least one of the reduced-diameter portions of the outer pipe is formed by spinning.
Further, this publication discloses a method of manufacturing an exhaust-system component characterized by comprising the steps of covering an inner pipe having substantially tapered reduced-diameter portions at both ends thereof with an outer pipe while maintaining an air gap therebetween, and spinning at least one end of the outer pipe to reduce its diameter in a substantially tapered manner. On the other hand, in order to ensure formation of the air gap, some double-pipe-structure catalyst containers of the related art have wire meshes 115 provided between an inner pipe 111 and an outer pipe 112 as shown in FIG. 10 or a bead formed therebetween.
Japanese Patent Application Laid-Open No. 6-101465 discloses a double-pipe catalytic converter characterized in that a catalyst casing (container) has a double-pipe structure composed of a first casing and a second casing to suppress a fall in temperature of the catalyst in a low-speed low-load range of an internal combustion engine, that an opening is provided in a most upstream portion in a casing gap between the first and second casings so that the casing gap communicates with an exhaust passage, that the catalyst is divided into a first catalyst and a second catalyst, that a hole for communication between the casing gap with a catalyst gap between the first and second catalysts is provided, and that an on-off valve is provided in the hole or the opening in the most upstream portion in the casing gap.
Further, in order to easily and inexpensively manufacture a double-structure container composed of an inner pipe and an outer pipe that have a predetermined gap therebetween and that have an integrally deformed cross-section, Japanese Patent Application Laid-Open No. 2000-94050 discloses a method of manufacturing a double-pipe-structure container characterized by comprising the steps of disposing an inner pipe inside an outer pipe while leaving a certain air gap therebetween and spinning the outer pipe with a solid interjacent material sandwiched in at least one axial section in the air gap to simultaneously deform cross-sections of the inner and outer pipes. In addition, this publication also discloses a construction wherein resonance holes are formed in the inner pipe in advance so that exhaust noise flows into an air gap of a resonance-type muffler section of a formed product through the resonance holes and that the air gap serves as a resonance space for causing a sound-deadening effect. Still further, as is also disclosed in this publication, a double-pipe-structure catalyst container of the related art has an inner pipe and an outer pipe that are fixed at one end thereof to other connecting pipes but that clamp at the other end thereof a wire-net ring for relatively displaceably holding the inner and outer pipes, so that relative displacement resulting from a difference in thermal expansion between the inner and outer pipes can be guaranteed.
Exhaust gas discharged from an internal combustion engine is at a high temperature and is introduced into a catalytic converter at a high flow rate while pulsating in accordance with combustion of the internal combustion engine. In the case where the catalyst container has a small thickness, since pulsating exhaust gas is introduced thereinto at a high flow rate, so-called radiation noise is generated especially due to vibrations of upstream cone portions connected with the internal combustion engine. To abate this radiation noise, it is contemplable to increase the characteristic frequency of the catalyst container. Further, the catalyst container is exposed to a high temperature and corroded through oxidization, and thus is required to demonstrate high temperature- and corrosion-resistance.
However, in the case of a single-pipe-structure catalyst container employing a hollow member according to the related art as described above, even though a thickness of e.g. 1 mm suffices for temperature- and corrosion-resistance and required rigidity for holding the catalyst, there is a problem of the necessity to set a thickness greater than the required thickness, e.g. 1.5 mm, for the purpose of increasing the characteristic frequency and guaranteeing high temperature- and corrosion-resistance with the intention of suppressing radiation noise. In the case of a double-pipe-structure catalyst container as described above, since the inner and outer pipes are independent from each other owing to an air gap formed therebetween, there is also a problem of the necessity to set a thickness greater than the required thickness in order to ensure sufficient rigidity, as is the case with the single-pipe-structure catalyst container.
Whether the catalyst container of the related art has a single-pipe structure or a double-pipe structure, there is a problem of the necessity to increase the overall thickness. This causes problems such as an increased amount of use of materials and inability to reduce the overall weight.
The invention has been made in view of the aforementioned problems. It is an object of the invention to provide a double-pipe-structure hollow member capable of maintaining the same rigidity as in the related art, achieving reduction in weight through a reduced thickness and being easily manufactured in a small number of steps, and a method of manufacturing such a double-pipe-structure hollow member. It is another object of the invention to provide a fluid treating system employing a double-pipe-structure hollow member capable of achieving reduction in weight through reduction in thickness of a double-pipe-structure hollow member employed in the fluid treating system.
The objects are achieved by a double-pipe-structure hollow member, a method of manufacturing a double-pipe-structure hollow member, and a fluid treating system employing a double-pipe-structure hollow member as described below.
A double-pipe-structure hollow member is composed of an inner pipe and an outer pipe that are concentrically superposed on each other such that an outer periphery of the inner pipe and an inner periphery of the outer pipe are in contact with each other. By spinning the concentrically disposed inner and outer pipes, a double-pipe-structure hollow member with the outer periphery of the inner pipe and the inner periphery of the outer pipe superposed on each other is manufactured. The double-pipe-structure hollow member thus manufactured is employed as a container for a fluid treating member, thus constituting a fluid treating system.
In such a double-pipe-structure hollow member wherein the inner and outer pipes are disposed concentrically to be spun, the inner and outer pipes are concentrically disposed and superposed on each other such that the outer periphery of the inner pipe and the inner periphery of the outer pipe are in contact with each other. Therefore, it is possible to obtain the same rigidity as of a hollow member of a thickness equal to or greater than the sum of the thickness of the inner pipe and the thickness of the outer pipe. Thus, the entire double-pipe-structure hollow member can be reduced in thickness and weight.
By employing the double-pipe-structure hollow member as a container for a fluid treating member and thus constituting a fluid treating system, the entire fluid treating system can be reduced in weight.